On 05/01/2014 01:50 PM, Tim Chen wrote:
It takes me a while to understand how rwsem's count field mainifest itself in different scenarios. I'm adding comments to provide a quick reference on the the rwsem's count field for each scenario where readers and writers are contending/holding the lock. Hopefully it will be useful for future maintenance of the code and for people to get up to speed on how the logic in the code works.
Except there are a lot of transition states for the count that look like stable states for some other condition, and vice versa. For example, 0xffff000X could be: 1. stable state as described below. 2. 1 or more (but not X) readers active, 1 writer which failed to acquire and has not yet backed out the adjustment 0 or more readers which failed to acquire because of the waiting writer and have not yet backed out 3. 1 writer active, 1 or more readers which failed to acquire because of the active writer and have not yet backed out 4. maybe more states where a owning writer has just dropped the lock Because of this, it's hazardous to infer lock state except for the specific existing tests (eg., the count observed by a failed reader after it has acquired the wait_lock). Regards, Peter Hurley
Signed-off-by: Tim Chen <tim.c.c...@linux.intel.com> --- kernel/locking/rwsem-xadd.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 1d66e08..305c15f 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -12,6 +12,28 @@ #include <linux/export.h> /* + * Meaning of the rw_semaphore's count field: + * (32 bit case illustrated, similar for 64 bit) + * + * (listed in decreasing order) + * 0x0000000X X readers active, no writer waiting (X*ACTIVE_BIAS) + * 0x00000000 rwsem is unlocked, and no one is waiting for the lock + * 0xffff000X X readers active, writer and/or reader waiting. + * (X*ACTIVE_BIAS + WAITING_BIAS) + * 0xffff0001 (1) one writer active, nobody queued. (ACTIVE_WRITE_BIAS) + * or + * (2) one reader active, writer(s) queued. + * (WAITING_BIAS + ACTIVE_BIAS) + * 0xffff0000 There are writers or readers queued but none active + * (WAITING_BIAS), a writer can try to grab the lock and + * take itself off wait list if awake. Writer who has just + * completed its task seeing this count will try to + * wake people up from wait list. + * 0xfffe0001 Writer active, readers/writer queued + * (ACTIVE_WRITE_BIAS + WAITING_BIAS) + */ + +/* * Initialize an rwsem: */ void __init_rwsem(struct rw_semaphore *sem, const char *name,
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