Re: Question about atomics

[Richard Henderson <richard.henderson@linaro.org>]

On 3/13/22 09:57, Warner Losh wrote:
> 
> 
> On Sun, Mar 13, 2022, 10:47 AM Richard Henderson <richard.henderson@linaro.org 
> <mailto:richard.henderson@linaro.org>> wrote:
> 
>     On 3/12/22 20:59, Warner Losh wrote:
>      > FreeBSD's pthread_mutex is shared between the kernel and user land.
>      > So it does a compare and set to take the lock. Uncontested and unheld
>      > locks will mean we've taken the lock and return. Contested locks
>      > are kicked to the kernel to wait. When userland releases the lock
>      > it signals the kernel to wakeup via a system call. The kernel then
>      > does a cas to try to acquire the lock. It either returns with the lock
>      > held, or goes back to sleep. This we have atomics operating both in
>      > the kernel (via standard host atomics) and userland atomics done
>      > via start/end_exclusive.
> 
>     You need to use standard host atomics for this case.
> 
> 
> Or use the start/end_exclusive for both by emulating the kernel call, I presume? It's the 
> mixing that's the problem, right?

Well, preferably no.  Use start/end_exclusive only when you have no alternative, which for 
a simple CAS should not be the case on any FreeBSD host.

Using start/end_exclusive is entirely local to the current process, and means you don't 
have atomicity across processes.  Which can cause problems when emulating an entire chroot.


r~