Messages in this thread | | | Date | Fri, 29 Mar 2019 15:30:57 -0700 | Subject | Re: Potentially missing "memory" clobbers in bitops.h for x86 | From | hpa@zytor ... |
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On March 29, 2019 3:05:54 PM PDT, "Paul E. McKenney" <paulmck@linux.ibm.com> wrote: >On Fri, Mar 29, 2019 at 02:51:26PM -0700, H. Peter Anvin wrote: >> On 3/29/19 2:09 PM, Paul E. McKenney wrote: >> >> >> >> Note: the atomic versions of these functions obviously need to >have >> >> "volatile" and the clobber anyway, as they are by definition >barriers >> >> and moving memory operations around them would be a very serious >error. >> > >> > The atomic functions that return void don't need to order anything >except >> > the input and output arguments. The oddness with clear_bit() is >that the >> > memory changed isn't necessarily the quantity referenced by the >argument, >> > if the number of bits specified is large. >> > >> > So (for example) atomic_inc() does not need a "memory" clobber, >right? >> >> I don't believe that is true: the code calling it has a reasonable >> expectation that previous memory operations have finished and later >> memory operations have not started from the point of view of another >> processor. You are more of an expert on memory ordering than I am, >but >> I'm 89% sure that there is plenty of code in the kernel which makes >that >> assumption. > >From Documentation/core-api/atomic_ops.rst: > >------------------------------------------------------------------------ > void atomic_add(int i, atomic_t *v); > void atomic_sub(int i, atomic_t *v); > void atomic_inc(atomic_t *v); > void atomic_dec(atomic_t *v); > >These four routines add and subtract integral values to/from the given >atomic_t value. The first two routines pass explicit integers by >which to make the adjustment, whereas the latter two use an implicit >adjustment value of "1". > >One very important aspect of these two routines is that they DO NOT >require any explicit memory barriers. They need only perform the >atomic_t counter update in an SMP safe manner. >------------------------------------------------------------------------ > >So, no, these functions do not imply any ordering other than to the >variable modified. This one predates my joining the Linux kernel >community. ;-) So any cases where someone is relying on atomic_inc() >to provide ordering are bugs. > >Now for value-returning atomics, for example, atomic_inc_return(), >full ordering is indeed required. > > Thanx, Paul
Ok. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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