On 9/27/23 05:04, Kemeng Shi wrote:
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!
Sounds good to me, thanks for reading the document carefully!
Bernd
