On 03/10/15 16:59, valdis.kletni...@vt.edu wrote:
On Tue, 10 Mar 2015 14:03:59 +0100, Yann Droneaud said:
Consider the following sequence of events:
0. Suppose a mutex is locked by task A and has no waiters.
1. Task B calls mutex_trylock().
2. mutex_trylock() calls the architecture-specific
__mutex_fastpath_trylock(), with __mutex_trylock_slowpath() as
fail_fn.
3. According to the description of __mutex_fastpath_trylock() (for
example in include/asm-generic/mutex-dec.h), "if the architecture
has no effective trylock variant, it should call the fail_fn
spinlock-based trylock variant unconditionally". So
__mutex_fastpath_trylock() may now call __mutex_trylock_slowpath().
4. Task A releases the mutex.
5. Task B, in __mutex_trylock_slowpath, executes:
/* No need to trylock if the mutex is locked. */
if (mutex_is_locked(lock))
return 0;
Since the mutex is no longer locked, the function continues.
6. Task C, which runs on a different cpu than task B, locks the mutex
again.
7. Task B, in __mutex_trylock_slowpath(), continues:
spin_lock_mutex(&lock->wait_lock, flags);
B will spin here until C releases the lock.
When that spin exits, C no longer holds the lock. Re-do the analysis
from this point.
Thank you for the review.
I don't think B waits for C here - C holds the mutex (lock), not the
internal spinlock (lock->wait_lock). I might be wrong though.
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