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SubjectRe: [PATCH v4 4/4] perf/core,x86: synchronize PMU task contexts on optimized context switches
On Tue, Oct 22, 2019 at 09:01:11AM +0300, Alexey Budankov wrote:

> swap(ctx->task_ctx_data, next_ctx->task_ctx_data);
>
> + /*
> + * PMU specific parts of task perf context can require
> + * additional synchronization which makes sense only if
> + * both next_ctx->task_ctx_data and ctx->task_ctx_data
> + * pointers are allocated. As an example of such
> + * synchronization see implementation details of Intel
> + * LBR call stack data profiling;
> + */
> + if (ctx->task_ctx_data && next_ctx->task_ctx_data)
> + pmu->sync_task_ctx(next_ctx->task_ctx_data,
> + ctx->task_ctx_data);

This still does not check if pmu->sync_task_ctx is set. If any other
arch ever uses task_ctx_data without then also supplying this method
things will go *bang*.

Also, I think I prefer the variant I gave you yesterday:

https://lkml.kernel.org/r/20191021103745.GF1800@hirez.programming.kicks-ass.net

if (pmu->swap_task_ctx)
pmu->swap_task_ctx(ctx, next_ctx);
else
swap(ctx->task_ctx_data, next_ctx->task_ctx_data);

That also unconfuses the argument order in your above patch (where you
have to undo thw swap).

Alternatively, since there currently is no other arch using
task_ctx_data, we can make the pmu::swap_task_ctx() thing mandatory when
having it and completely replace the swap(), write it like so:


- swap(ctx->task_ctx_data, next_ctx->task_ctx_data);
+ if (pmu->swap_task_ctx)
+ pmu->swap_task_ctx(ctx, next_ctx);

Hmm?

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