Messages in this thread | | | Date | Tue, 19 Sep 2023 15:12:13 +0200 | Subject | Re: [PATCH] fuse: remove unneeded lock which protecting update of congestion_threshold | From | Bernd Schubert <> |
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On 9/19/23 08:11, Kemeng Shi wrote: > > > on 9/16/2023 7:06 PM, Bernd Schubert wrote: >> >> >> On 9/14/23 17:45, Kemeng Shi wrote: >>> Commit 670d21c6e17f6 ("fuse: remove reliance on bdi congestion") change how >>> congestion_threshold is used and lock in >>> fuse_conn_congestion_threshold_write is not needed anymore. >>> 1. Access to supe_block is removed along with removing of bdi congestion. >>> Then down_read(&fc->killsb) which protecting access to super_block is no >>> needed. >>> 2. Compare num_background and congestion_threshold without holding >>> bg_lock. Then there is no need to hold bg_lock to update >>> congestion_threshold. >>> >>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com> >>> --- >>> fs/fuse/control.c | 4 ---- >>> 1 file changed, 4 deletions(-) >>> >>> diff --git a/fs/fuse/control.c b/fs/fuse/control.c >>> index 247ef4f76761..c5d7bf80efed 100644 >>> --- a/fs/fuse/control.c >>> +++ b/fs/fuse/control.c >>> @@ -174,11 +174,7 @@ static ssize_t fuse_conn_congestion_threshold_write(struct file *file, >>> if (!fc) >>> goto out; >>> - down_read(&fc->killsb); >>> - spin_lock(&fc->bg_lock); >>> fc->congestion_threshold = val; >>> - spin_unlock(&fc->bg_lock); >>> - up_read(&fc->killsb); >>> fuse_conn_put(fc); >>> out: >>> return ret; >> >> Yeah, I don't see readers holding any of these locks. >> I just wonder if it wouldn't be better to use WRITE_ONCE to ensure a single atomic operation to store the value. > Sure, WRITE_ONCE looks better. I wonder if we should use READ_ONCE from reader. > Would like to get any advice. Thanks!
I'm not entirely sure either, but I _think_ the compiler is free to store a 32 bit value with multiple operations (like 2 x 16 bit). In that case a competing reading thread might read garbage... Although I don't see this documented here https://www.kernel.org/doc/Documentation/memory-barriers.txt Though documented there is that the compile is free to optimize out the storage at all, see "(*) Similarly, the compiler is within its rights to omit a store entirely"
Regarding READ_ONCE, I don't have a strong opinion, if the compiler makes some optimizations and the value would be wrong for a few cycles, would that matter for that variable? Unless the compiler would be really creative and the variable would get never updated... For sure READ_ONCE would be safer, but I don't know if it is needed SSee section "The compiler is within its rights to omit a load entirely if it know" in the document above.
Thanks, Bernd
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