on 9/19/2023 9:12 PM, Bernd Schubert wrote: > > > 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@xxxxxxxxxxxxxxx> >>>> --- >>>> 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! > Sorry for the dealy - it toke me some time to go through the barrier documents. > 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 I found this is documented in section "(*) For aligned memory locations whose size allows them to be accessed..." Then WRITE_ONCE is absolutely needed now as you menthioned before. > 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. I go through all examples of optimizations in document and congestion_threshold has no same trouble descripted in document. For specifc case you menthioned that "The compiler is within its rights to omit a load entirely if it know". The compiler will keep the first load and only omit successive loads from same variable in loop. As there is no repeat loading from congestion_threshold in loop, congestion_threshold is out of this trouble. Anyway, congestion_threshold is more like a hint and the worst case is that it is stale for a few cycles. I prefer to keep reading congestion_threshold without READ_ONCE and will do it in next version if it's fine to you. Thanks! > > Thanks, > Bernd > > > >