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refcount_t: Add ACQUIRE ordering on success for dec(sub)_and_test() variants

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This adds an smp_acquire__after_ctrl_dep() barrier on successful
decrease of refcounter value from 1 to 0 for refcount_dec(sub)_and_test
variants and therefore gives stronger memory ordering guarantees than
prior versions of these functions.

Co-developed-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Elena Reshetova <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Andrea Parri <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Elena Reshetova 2019-01-30 13:18:51 +02:00 • committed by Ingo Molnar
commit 47b8f3ab9c
3 changed files with 52 additions and 12 deletions
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18
lib/refcount.c
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@ -33,6 +33,9 @@
* Note that the allocator is responsible for ordering things between free()
* and alloc().
*
* The decrements dec_and_test() and sub_and_test() also provide acquire
* ordering on success.
*
*/
#include <linux/mutex.h>
@ -164,8 +167,8 @@ EXPORT_SYMBOL(refcount_inc_checked);
* at UINT_MAX.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides a control dependency such that free() must come after.
* See the comment on top.
* before, and provides an acquire ordering on success such that free()
* must come after.
*
* Use of this function is not recommended for the normal reference counting
* use case in which references are taken and released one at a time. In these
@ -190,7 +193,12 @@ bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r)
} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
return !new;
if (!new) {
smp_acquire__after_ctrl_dep();
return true;
}
return false;
}
EXPORT_SYMBOL(refcount_sub_and_test_checked);
@ -202,8 +210,8 @@ EXPORT_SYMBOL(refcount_sub_and_test_checked);
* decrement when saturated at UINT_MAX.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides a control dependency such that free() must come after.
* See the comment on top.
* before, and provides an acquire ordering on success such that free()
* must come after.
*
* Return: true if the resulting refcount is 0, false otherwise
*/