Re: [PATCH] aio-posix: fix spurious ->poll_ready() callbacks in main loop

2022-03-01 Thread Stefan Hajnoczi 
On Wed, Feb 23, 2022 at 03:57:03PM +, Stefan Hajnoczi wrote:
> When ->poll() succeeds the AioHandler is placed on the ready list with
> revents set to the magic value 0. This magic value causes
> aio_dispatch_handler() to invoke ->poll_ready() instead of ->io_read()
> for G_IO_IN or ->io_write() for G_IO_OUT.
> 
> This magic value 0 hack works for the IOThread where AioHandlers are
> placed on ->ready_list and processed by aio_dispatch_ready_handlers().
> It does not work for the main loop where all AioHandlers are processed
> by aio_dispatch_handlers(), even those that are not ready and have a
> revents value of 0.
> 
> As a result the main loop invokes ->poll_ready() on AioHandlers that are
> not ready. These spurious ->poll_ready() calls waste CPU cycles and
> could lead to crashes if the code assumes ->poll() must have succeeded
> before ->poll_ready() is called (a reasonable asumption but I haven't
> seen it in practice).
> 
> Stop using revents to track whether ->poll_ready() will be called on an
> AioHandler. Introduce a separate AioHandler->poll_ready field instead.
> This eliminates spurious ->poll_ready() calls in the main loop.
> 
> Fixes: 826cc32423db2a99d184dbf4f507c737d7e7a4ae ("aio-posix: split poll check 
> from ready handler")
> Signed-off-by: Stefan Hajnoczi 
> ---
>  util/aio-posix.h |  1 +
>  util/aio-posix.c | 32 ++--
>  2 files changed, 19 insertions(+), 14 deletions(-)

Applied to my block tree:
https://gitlab.com/stefanha/qemu/commits/block

Stefan


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Re: [PATCH] aio-posix: fix spurious ->poll_ready() callbacks in main loop

2022-02-24 Thread Jason Wang 



在 2022/2/23 下午11:57, Stefan Hajnoczi 写道:

When ->poll() succeeds the AioHandler is placed on the ready list with
revents set to the magic value 0. This magic value causes
aio_dispatch_handler() to invoke ->poll_ready() instead of ->io_read()
for G_IO_IN or ->io_write() for G_IO_OUT.

This magic value 0 hack works for the IOThread where AioHandlers are
placed on ->ready_list and processed by aio_dispatch_ready_handlers().
It does not work for the main loop where all AioHandlers are processed
by aio_dispatch_handlers(), even those that are not ready and have a
revents value of 0.

As a result the main loop invokes ->poll_ready() on AioHandlers that are
not ready. These spurious ->poll_ready() calls waste CPU cycles and
could lead to crashes if the code assumes ->poll() must have succeeded
before ->poll_ready() is called (a reasonable asumption but I haven't
seen it in practice).

Stop using revents to track whether ->poll_ready() will be called on an
AioHandler. Introduce a separate AioHandler->poll_ready field instead.
This eliminates spurious ->poll_ready() calls in the main loop.

Fixes: 826cc32423db2a99d184dbf4f507c737d7e7a4ae ("aio-posix: split poll check from 
ready handler")
Signed-off-by: Stefan Hajnoczi 



Reported-by: Jason Wang 

Tested-by: Jason Wang 

Thanks



---
  util/aio-posix.h |  1 +
  util/aio-posix.c | 32 ++--
  2 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/util/aio-posix.h b/util/aio-posix.h
index 7f2c37a684..80b927c7f4 100644
--- a/util/aio-posix.h
+++ b/util/aio-posix.h
@@ -37,6 +37,7 @@ struct AioHandler {
  unsigned flags; /* see fdmon-io_uring.c */
  #endif
  int64_t poll_idle_timeout; /* when to stop userspace polling */
+bool poll_ready; /* has polling detected an event? */
  bool is_external;
  };
  
diff --git a/util/aio-posix.c b/util/aio-posix.c

index 7b9f629218..be0182a3c6 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -23,15 +23,6 @@
  #include "trace.h"
  #include "aio-posix.h"
  
-/*

- * G_IO_IN and G_IO_OUT are not appropriate revents values for polling, since
- * the handler may not need to access the file descriptor. For example, the
- * handler doesn't need to read from an EventNotifier if it polled a memory
- * location and a read syscall would be slow. Define our own unique revents
- * value to indicate that polling determined this AioHandler is ready.
- */
-#define REVENTS_POLL_READY 0
-
  /* Stop userspace polling on a handler if it isn't active for some time */
  #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
  
@@ -49,6 +40,14 @@ void aio_add_ready_handler(AioHandlerList *ready_list,

  QLIST_INSERT_HEAD(ready_list, node, node_ready);
  }
  
+static void aio_add_poll_ready_handler(AioHandlerList *ready_list,

+   AioHandler *node)
+{
+QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
+node->poll_ready = true;
+QLIST_INSERT_HEAD(ready_list, node, node_ready);
+}
+
  static AioHandler *find_aio_handler(AioContext *ctx, int fd)
  {
  AioHandler *node;
@@ -76,6 +75,7 @@ static bool aio_remove_fd_handler(AioContext *ctx, AioHandler 
*node)
  }
  
  node->pfd.revents = 0;

+node->poll_ready = false;
  
  /* If the fd monitor has already marked it deleted, leave it alone */

  if (QLIST_IS_INSERTED(node, node_deleted)) {
@@ -247,7 +247,7 @@ static bool poll_set_started(AioContext *ctx, 
AioHandlerList *ready_list,
  
  /* Poll one last time in case ->io_poll_end() raced with the event */

  if (!started && node->io_poll(node->opaque)) {
-aio_add_ready_handler(ready_list, node, REVENTS_POLL_READY);
+aio_add_poll_ready_handler(ready_list, node);
  progress = true;
  }
  }
@@ -282,6 +282,7 @@ bool aio_pending(AioContext *ctx)
  QLIST_FOREACH_RCU(node, >aio_handlers, node) {
  int revents;
  
+/* TODO should this check poll ready? */

  revents = node->pfd.revents & node->pfd.events;
  if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
  aio_node_check(ctx, node->is_external)) {
@@ -323,11 +324,15 @@ static void aio_free_deleted_handlers(AioContext *ctx)
  static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
  {
  bool progress = false;
+bool poll_ready;
  int revents;
  
  revents = node->pfd.revents & node->pfd.events;

  node->pfd.revents = 0;
  
+poll_ready = node->poll_ready;

+node->poll_ready = false;
+
  /*
   * Start polling AioHandlers when they become ready because activity is
   * likely to continue.  Note that starvation is theoretically possible 
when
@@ -344,7 +349,7 @@ static bool aio_dispatch_handler(AioContext *ctx, 
AioHandler *node)
  QLIST_INSERT_HEAD(>poll_aio_handlers, node, node_poll);
  }
  if (!QLIST_IS_INSERTED(node, node_deleted) &&
-revents == 0 &&
+

[PATCH] aio-posix: fix spurious ->poll_ready() callbacks in main loop

2022-02-23 Thread Stefan Hajnoczi 
When ->poll() succeeds the AioHandler is placed on the ready list with
revents set to the magic value 0. This magic value causes
aio_dispatch_handler() to invoke ->poll_ready() instead of ->io_read()
for G_IO_IN or ->io_write() for G_IO_OUT.

This magic value 0 hack works for the IOThread where AioHandlers are
placed on ->ready_list and processed by aio_dispatch_ready_handlers().
It does not work for the main loop where all AioHandlers are processed
by aio_dispatch_handlers(), even those that are not ready and have a
revents value of 0.

As a result the main loop invokes ->poll_ready() on AioHandlers that are
not ready. These spurious ->poll_ready() calls waste CPU cycles and
could lead to crashes if the code assumes ->poll() must have succeeded
before ->poll_ready() is called (a reasonable asumption but I haven't
seen it in practice).

Stop using revents to track whether ->poll_ready() will be called on an
AioHandler. Introduce a separate AioHandler->poll_ready field instead.
This eliminates spurious ->poll_ready() calls in the main loop.

Fixes: 826cc32423db2a99d184dbf4f507c737d7e7a4ae ("aio-posix: split poll check 
from ready handler")
Signed-off-by: Stefan Hajnoczi 
---
 util/aio-posix.h |  1 +
 util/aio-posix.c | 32 ++--
 2 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/util/aio-posix.h b/util/aio-posix.h
index 7f2c37a684..80b927c7f4 100644
--- a/util/aio-posix.h
+++ b/util/aio-posix.h
@@ -37,6 +37,7 @@ struct AioHandler {
 unsigned flags; /* see fdmon-io_uring.c */
 #endif
 int64_t poll_idle_timeout; /* when to stop userspace polling */
+bool poll_ready; /* has polling detected an event? */
 bool is_external;
 };
 
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 7b9f629218..be0182a3c6 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -23,15 +23,6 @@
 #include "trace.h"
 #include "aio-posix.h"
 
-/*
- * G_IO_IN and G_IO_OUT are not appropriate revents values for polling, since
- * the handler may not need to access the file descriptor. For example, the
- * handler doesn't need to read from an EventNotifier if it polled a memory
- * location and a read syscall would be slow. Define our own unique revents
- * value to indicate that polling determined this AioHandler is ready.
- */
-#define REVENTS_POLL_READY 0
-
 /* Stop userspace polling on a handler if it isn't active for some time */
 #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
 
@@ -49,6 +40,14 @@ void aio_add_ready_handler(AioHandlerList *ready_list,
 QLIST_INSERT_HEAD(ready_list, node, node_ready);
 }
 
+static void aio_add_poll_ready_handler(AioHandlerList *ready_list,
+   AioHandler *node)
+{
+QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
+node->poll_ready = true;
+QLIST_INSERT_HEAD(ready_list, node, node_ready);
+}
+
 static AioHandler *find_aio_handler(AioContext *ctx, int fd)
 {
 AioHandler *node;
@@ -76,6 +75,7 @@ static bool aio_remove_fd_handler(AioContext *ctx, AioHandler 
*node)
 }
 
 node->pfd.revents = 0;
+node->poll_ready = false;
 
 /* If the fd monitor has already marked it deleted, leave it alone */
 if (QLIST_IS_INSERTED(node, node_deleted)) {
@@ -247,7 +247,7 @@ static bool poll_set_started(AioContext *ctx, 
AioHandlerList *ready_list,
 
 /* Poll one last time in case ->io_poll_end() raced with the event */
 if (!started && node->io_poll(node->opaque)) {
-aio_add_ready_handler(ready_list, node, REVENTS_POLL_READY);
+aio_add_poll_ready_handler(ready_list, node);
 progress = true;
 }
 }
@@ -282,6 +282,7 @@ bool aio_pending(AioContext *ctx)
 QLIST_FOREACH_RCU(node, >aio_handlers, node) {
 int revents;
 
+/* TODO should this check poll ready? */
 revents = node->pfd.revents & node->pfd.events;
 if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
 aio_node_check(ctx, node->is_external)) {
@@ -323,11 +324,15 @@ static void aio_free_deleted_handlers(AioContext *ctx)
 static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
 {
 bool progress = false;
+bool poll_ready;
 int revents;
 
 revents = node->pfd.revents & node->pfd.events;
 node->pfd.revents = 0;
 
+poll_ready = node->poll_ready;
+node->poll_ready = false;
+
 /*
  * Start polling AioHandlers when they become ready because activity is
  * likely to continue.  Note that starvation is theoretically possible when
@@ -344,7 +349,7 @@ static bool aio_dispatch_handler(AioContext *ctx, 
AioHandler *node)
 QLIST_INSERT_HEAD(>poll_aio_handlers, node, node_poll);
 }
 if (!QLIST_IS_INSERTED(node, node_deleted) &&
-revents == 0 &&
+poll_ready && revents == 0 &&
 aio_node_check(ctx, node->is_external) &&
 node->io_poll_ready) {
 node->io_poll_ready(node->opaque);
@@ -432,7