https://gcc.gnu.org/g:8a47ae5c193bfc5d84f90a41b122f83b9cd6d691
commit r16-6912-g8a47ae5c193bfc5d84f90a41b122f83b9cd6d691 Author: Matthew Malcolmson <[email protected]> Date: Tue Jan 20 03:29:04 2026 +0000 libgomp: Enforce tasks executed lexically after scheduled In PR122314 we noticed that our implementation of a barrier could execute tasks from the next "Task scheduling" region. This was because of a race condition where a barrier could be "completed", and some thread raced ahead to schedule another task on the "next" barrier all before some other thread checks for a bit on the generation number to tell if there is a task pending. The solution provided here is to check whether the generation number has "incremented" past the state that this barrier was entered with. As it happens the `state` variable already provided to `gomp_barrier_handle_tasks` is enough for the targets to tell whether the current global generation has incremented from the existing one. This requires some changes in the two loops in bar.c that are waiting on tasks being available. These loops now need to check for "generation has incremented" rather than "generation is identical to one increment forward". Without such an adjustment of the check a thread that is refusing to execute tasks because they have been scheduled for the next barrier will not continue into the next region until some other thread has completed the task (and removed the BAR_TASK_PENDING flag). This problem could be seen by a hang in testcases like task-reduction-13.c. Testing done: - Bootstrap & regtest on aarch64 and x86_64. - With & without _LIBGOMP_CHECKING_. - Testsuite with & without OMP_WAIT_POLICY=passive - Cross compilation & regtest on arm. - TSAN done on this as part of all my upstream patches. libgomp/ChangeLog: PR libgomp/122314 PR libgomp/88707 * config/gcn/bar.c (gomp_team_barrier_wait_end): Use gomp_barrier_state_is_incremented. (gomp_team_barrier_wait_cancel_end): Likewise. * config/gcn/bar.h (gomp_barrier_state_is_incremented, gomp_barrier_has_completed): New. * config/linux/bar.c (gomp_team_barrier_wait_end): Use gomp_barrier_state_is_incremented. (gomp_team_barrier_wait_cancel_end): Likewise. * config/linux/bar.h (gomp_barrier_state_is_incremented, gomp_barrier_has_completed): New. * config/nvptx/bar.h (gomp_barrier_state_is_incremented, gomp_barrier_has_completed): New. * config/posix/bar.c (gomp_team_barrier_wait_end): Use gomp_barrier_state_is_incremented. (gomp_team_barrier_wait_cancel_end): Likewise * config/posix/bar.h (gomp_barrier_state_is_incremented, gomp_barrier_has_completed): New. * config/rtems/bar.h (gomp_barrier_state_is_incremented, gomp_barrier_has_completed): New. * task.c (gomp_barrier_handle_tasks): Use gomp_barrier_has_completed. * testsuite/libgomp.c/pr122314.c: New test. Signed-off-by: Matthew Malcomson <[email protected]> Diff: --- libgomp/config/gcn/bar.c | 4 ++-- libgomp/config/gcn/bar.h | 16 +++++++++++++ libgomp/config/linux/bar.c | 4 ++-- libgomp/config/linux/bar.h | 16 +++++++++++++ libgomp/config/nvptx/bar.h | 16 +++++++++++++ libgomp/config/posix/bar.c | 4 ++-- libgomp/config/posix/bar.h | 16 +++++++++++++ libgomp/config/rtems/bar.h | 16 +++++++++++++ libgomp/task.c | 17 ++++++++++++++ libgomp/testsuite/libgomp.c/pr122314.c | 42 ++++++++++++++++++++++++++++++++++ 10 files changed, 145 insertions(+), 6 deletions(-) diff --git a/libgomp/config/gcn/bar.c b/libgomp/config/gcn/bar.c index a655015f612d..10c3f5d13623 100644 --- a/libgomp/config/gcn/bar.c +++ b/libgomp/config/gcn/bar.c @@ -128,7 +128,7 @@ gomp_team_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state) gen = __atomic_load_n (&bar->generation, MEMMODEL_ACQUIRE); } } - while (gen != state + BAR_INCR); + while (!gomp_barrier_state_is_incremented (gen, state)); } void @@ -207,7 +207,7 @@ gomp_team_barrier_wait_cancel_end (gomp_barrier_t *bar, gen = __atomic_load_n (&bar->generation, MEMMODEL_RELAXED); } } - while (gen != state + BAR_INCR); + while (!gomp_barrier_state_is_incremented (gen, state)); return false; } diff --git a/libgomp/config/gcn/bar.h b/libgomp/config/gcn/bar.h index 4df46960470e..0507efb7d2d1 100644 --- a/libgomp/config/gcn/bar.h +++ b/libgomp/config/gcn/bar.h @@ -165,4 +165,20 @@ gomp_team_barrier_done (gomp_barrier_t *bar, gomp_barrier_state_t state) bar->generation = (state & -BAR_INCR) + BAR_INCR; } +static inline bool +gomp_barrier_state_is_incremented (gomp_barrier_state_t gen, + gomp_barrier_state_t state) +{ + unsigned next_state = (state & -BAR_INCR) + BAR_INCR; + return next_state > state ? gen >= next_state : gen < state; +} + +static inline bool +gomp_barrier_has_completed (gomp_barrier_state_t state, gomp_barrier_t *bar) +{ + /* Handling overflow in the generation. The "next" state could be less than + or greater than the current one. */ + return gomp_barrier_state_is_incremented (bar->generation, state); +} + #endif /* GOMP_BARRIER_H */ diff --git a/libgomp/config/linux/bar.c b/libgomp/config/linux/bar.c index e850cebb51f8..2a1b052b11e6 100644 --- a/libgomp/config/linux/bar.c +++ b/libgomp/config/linux/bar.c @@ -118,7 +118,7 @@ gomp_team_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state) } generation |= gen & BAR_WAITING_FOR_TASK; } - while (gen != state + BAR_INCR); + while (!gomp_barrier_state_is_incremented (gen, state)); } void @@ -185,7 +185,7 @@ gomp_team_barrier_wait_cancel_end (gomp_barrier_t *bar, } generation |= gen & BAR_WAITING_FOR_TASK; } - while (gen != state + BAR_INCR); + while (!gomp_barrier_state_is_incremented (gen, state)); return false; } diff --git a/libgomp/config/linux/bar.h b/libgomp/config/linux/bar.h index 3ad3111f3ddb..b1fff01105a7 100644 --- a/libgomp/config/linux/bar.h +++ b/libgomp/config/linux/bar.h @@ -165,4 +165,20 @@ gomp_team_barrier_done (gomp_barrier_t *bar, gomp_barrier_state_t state) bar->generation = (state & -BAR_INCR) + BAR_INCR; } +static inline bool +gomp_barrier_state_is_incremented (gomp_barrier_state_t gen, + gomp_barrier_state_t state) +{ + unsigned next_state = (state & -BAR_INCR) + BAR_INCR; + return next_state > state ? gen >= next_state : gen < state; +} + +static inline bool +gomp_barrier_has_completed (gomp_barrier_state_t state, gomp_barrier_t *bar) +{ + /* Handling overflow in the generation. The "next" state could be less than + or greater than the current one. */ + return gomp_barrier_state_is_incremented (bar->generation, state); +} + #endif /* GOMP_BARRIER_H */ diff --git a/libgomp/config/nvptx/bar.h b/libgomp/config/nvptx/bar.h index 2ec1eb0f39b3..aa2592ba5b3f 100644 --- a/libgomp/config/nvptx/bar.h +++ b/libgomp/config/nvptx/bar.h @@ -169,4 +169,20 @@ gomp_team_barrier_done (gomp_barrier_t *bar, gomp_barrier_state_t state) bar->generation = (state & -BAR_INCR) + BAR_INCR; } +static inline bool +gomp_barrier_state_is_incremented (gomp_barrier_state_t gen, + gomp_barrier_state_t state) +{ + unsigned next_state = (state & -BAR_INCR) + BAR_INCR; + return next_state > state ? gen >= next_state : gen < state; +} + +static inline bool +gomp_barrier_has_completed (gomp_barrier_state_t state, gomp_barrier_t *bar) +{ + /* Handling overflow in the generation. The "next" state could be less than + or greater than the current one. */ + return gomp_barrier_state_is_incremented (bar->generation, state); +} + #endif /* GOMP_BARRIER_H */ diff --git a/libgomp/config/posix/bar.c b/libgomp/config/posix/bar.c index 31451cd8bfa7..ce69905ba674 100644 --- a/libgomp/config/posix/bar.c +++ b/libgomp/config/posix/bar.c @@ -156,7 +156,7 @@ gomp_team_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state) gen = __atomic_load_n (&bar->generation, MEMMODEL_ACQUIRE); } } - while (gen != state + BAR_INCR); + while (!gomp_barrier_state_is_incremented (gen, state)); #ifdef HAVE_SYNC_BUILTINS n = __sync_add_and_fetch (&bar->arrived, -1); @@ -228,7 +228,7 @@ gomp_team_barrier_wait_cancel_end (gomp_barrier_t *bar, break; } } - while (gen != state + BAR_INCR); + while (!gomp_barrier_state_is_incremented (gen, state)); #ifdef HAVE_SYNC_BUILTINS n = __sync_add_and_fetch (&bar->arrived, -1); diff --git a/libgomp/config/posix/bar.h b/libgomp/config/posix/bar.h index 33d25592daa2..5a175c228c26 100644 --- a/libgomp/config/posix/bar.h +++ b/libgomp/config/posix/bar.h @@ -155,4 +155,20 @@ gomp_team_barrier_done (gomp_barrier_t *bar, gomp_barrier_state_t state) bar->generation = (state & -BAR_INCR) + BAR_INCR; } +static inline bool +gomp_barrier_state_is_incremented (gomp_barrier_state_t gen, + gomp_barrier_state_t state) +{ + unsigned next_state = (state & -BAR_INCR) + BAR_INCR; + return next_state > state ? gen >= next_state : gen < state; +} + +static inline bool +gomp_barrier_has_completed (gomp_barrier_state_t state, gomp_barrier_t *bar) +{ + /* Handling overflow in the generation. The "next" state could be less than + or greater than the current one. */ + return gomp_barrier_state_is_incremented (bar->generation, state); +} + #endif /* GOMP_BARRIER_H */ diff --git a/libgomp/config/rtems/bar.h b/libgomp/config/rtems/bar.h index 27326db9c77c..61fa91f300f2 100644 --- a/libgomp/config/rtems/bar.h +++ b/libgomp/config/rtems/bar.h @@ -167,4 +167,20 @@ gomp_team_barrier_done (gomp_barrier_t *bar, gomp_barrier_state_t state) bar->generation = (state & -BAR_INCR) + BAR_INCR; } +static inline bool +gomp_barrier_state_is_incremented (gomp_barrier_state_t gen, + gomp_barrier_state_t state) +{ + unsigned next_state = (state & -BAR_INCR) + BAR_INCR; + return next_state > state ? gen >= next_state : gen < state; +} + +static inline bool +gomp_barrier_has_completed (gomp_barrier_state_t state, gomp_barrier_t *bar) +{ + /* Handling overflow in the generation. The "next" state could be less than + or greater than the current one. */ + return gomp_barrier_state_is_incremented (bar->generation, state); +} + #endif /* GOMP_BARRIER_H */ diff --git a/libgomp/task.c b/libgomp/task.c index a6f21b05687f..554636aadd5c 100644 --- a/libgomp/task.c +++ b/libgomp/task.c @@ -1559,6 +1559,23 @@ gomp_barrier_handle_tasks (gomp_barrier_state_t state) int do_wake = 0; gomp_mutex_lock (&team->task_lock); + /* Avoid running tasks from next task scheduling region (PR122314). + N.b. we check that `team->task_count != 0` in order to avoid the + non-atomic read of `bar->generation` "conflicting" (in the C standard + sense) with the atomic write of `bar->generation` in + `gomp_team_barrier_wait_end`. That conflict would otherwise be a + data-race and hence UB. One alternate approach could have been to + atomically load `bar->generation` in `gomp_barrier_has_completed`. + + When `task_count == 0` we're not going to perform tasks anyway, so the + problem of PR122314 is naturally avoided. */ + if (team->task_count != 0 + && gomp_barrier_has_completed (state, &team->barrier)) + { + gomp_mutex_unlock (&team->task_lock); + return; + } + if (gomp_barrier_last_thread (state)) { if (team->task_count == 0) diff --git a/libgomp/testsuite/libgomp.c/pr122314.c b/libgomp/testsuite/libgomp.c/pr122314.c new file mode 100644 index 000000000000..bb9565de7266 --- /dev/null +++ b/libgomp/testsuite/libgomp.c/pr122314.c @@ -0,0 +1,42 @@ +#include <omp.h> + +void abort (); + +#define NUM_THREADS 8 +unsigned full_data[NUM_THREADS] = {0}; +#pragma omp declare target enter(full_data) + +void +test () +{ +#pragma omp parallel num_threads(8) + { +#pragma omp barrier + /* Initialise so that if tasks are performed on the previous barrier their + updates get overridden. This is a key behaviour of this test. */ + full_data[omp_get_thread_num ()] = 0; +#pragma omp for + for (int i = 0; i < 10; i++) +#pragma omp task + { + full_data[omp_get_thread_num ()] += 1; + } + } + + unsigned total = 0; + for (int i = 0; i < NUM_THREADS; i++) + total += full_data[i]; + + if (total != 10) + abort (); +} +#pragma omp declare target enter(test) + +int +main () +{ + test (); + +#pragma omp target + test (); +}
