6.6-stable review patch. If anyone has any objections, please let me know.
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From: NeilBrown neilb@suse.de
[ Upstream commit 52d633def56c10fe3e82a2c5d88c3ecb3f4e4852 ]
There are common patterns in the kernel of using test_and_clear_bit() before wake_up_bit(), and atomic_dec_and_test() before wake_up_var().
These combinations don't need extra barriers but sometimes include them unnecessarily.
To help avoid the unnecessary barriers and to help discourage the general use of wake_up_bit/var (which is a fragile interface) introduce two combined functions which implement these patterns.
Also add store_release_wake_up() which supports the task of simply setting a non-atomic variable and sending a wakeup. This pattern requires barriers which are often omitted.
Signed-off-by: NeilBrown neilb@suse.de Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lore.kernel.org/r/20240925053405.3960701-5-neilb@suse.de Stable-dep-of: 1db3a48e83bb ("NFS: Fix wakeup of __nfs_lookup_revalidate() in unblock_revalidate()") Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/wait_bit.h | 60 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 60 insertions(+)
diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h index 7725b7579b78..2209c227e859 100644 --- a/include/linux/wait_bit.h +++ b/include/linux/wait_bit.h @@ -335,4 +335,64 @@ static inline void clear_and_wake_up_bit(int bit, void *word) wake_up_bit(word, bit); }
+/** + * test_and_clear_wake_up_bit - clear a bit if it was set: wake up anyone waiting on that bit + * @bit: the bit of the word being waited on + * @word: the address of memory containing that bit + * + * If the bit is set and can be atomically cleared, any tasks waiting in + * wait_on_bit() or similar will be woken. This call has the same + * complete ordering semantics as test_and_clear_bit(). Any changes to + * memory made before this call are guaranteed to be visible after the + * corresponding wait_on_bit() completes. + * + * Returns %true if the bit was successfully set and the wake up was sent. + */ +static inline bool test_and_clear_wake_up_bit(int bit, unsigned long *word) +{ + if (!test_and_clear_bit(bit, word)) + return false; + /* no extra barrier required */ + wake_up_bit(word, bit); + return true; +} + +/** + * atomic_dec_and_wake_up - decrement an atomic_t and if zero, wake up waiters + * @var: the variable to dec and test + * + * Decrements the atomic variable and if it reaches zero, send a wake_up to any + * processes waiting on the variable. + * + * This function has the same complete ordering semantics as atomic_dec_and_test. + * + * Returns %true is the variable reaches zero and the wake up was sent. + */ + +static inline bool atomic_dec_and_wake_up(atomic_t *var) +{ + if (!atomic_dec_and_test(var)) + return false; + /* No extra barrier required */ + wake_up_var(var); + return true; +} + +/** + * store_release_wake_up - update a variable and send a wake_up + * @var: the address of the variable to be updated and woken + * @val: the value to store in the variable. + * + * Store the given value in the variable send a wake up to any tasks + * waiting on the variable. All necessary barriers are included to ensure + * the task calling wait_var_event() sees the new value and all values + * written to memory before this call. + */ +#define store_release_wake_up(var, val) \ +do { \ + smp_store_release(var, val); \ + smp_mb(); \ + wake_up_var(var); \ +} while (0) + #endif /* _LINUX_WAIT_BIT_H */