On Mon, 2008-09-08 at 13:10 +0200, Andi Kleen wrote:
> I did some btrfs RTFS over the weeking and I have a hard time understanding
> what this code is attempting to do:
>
> 28 int btrfs_tree_lock(struct extent_buffer *eb)
> 29 {
> 30 int i;
> 31
> 32 if (mutex_trylock(&eb->mutex))
> 33 return 0;
> 34 for (i = 0; i < 512; i++) {
> 35 cpu_relax();
> 36 if (mutex_trylock(&eb->mutex))
> 37 return 0;
> 38 }
> 39 cpu_relax();
> 40 mutex_lock_nested(&eb->mutex, BTRFS_MAX_LEVEL -
> btrfs_header_level(e b));
> 41 return 0;
> 42 }
>
> The trylocks seem pretty pointless.
> I presume it can be all replaced with the mutex_lock_nested() in line 40.
> Also the return value seems pointless because noone checks it. Like
> in the appended patch. Or do I miss something?
The idea is to try to spin for a bit to avoid scheduling away, which is
especially important for the high levels. Most holders of the mutex
let it go very quickly.
-chris
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