• Rob Gardner's avatar
    sparc64: Prevent perf from running during super critical sections · fc290a11
    Rob Gardner authored
    This fixes another cause of random segfaults and bus errors that may
    occur while running perf with the callgraph option.
    
    Critical sections beginning with spin_lock_irqsave() raise the interrupt
    level to PIL_NORMAL_MAX (14) and intentionally do not block performance
    counter interrupts, which arrive at PIL_NMI (15).
    
    But some sections of code are "super critical" with respect to perf
    because the perf_callchain_user() path accesses user space and may cause
    TLB activity as well as faults as it unwinds the user stack.
    
    One particular critical section occurs in switch_mm:
    
            spin_lock_irqsave(&mm->context.lock, flags);
            ...
            load_secondary_context(mm);
            tsb_context_switch(mm);
            ...
            spin_unlock_irqrestore(&mm->context.lock, flags);
    
    If a perf interrupt arrives in between load_secondary_context() and
    tsb_context_switch(), then perf_callchain_user() could execute with
    the context ID of one process, but with an active TSB for a different
    process. When the user stack is accessed, it is very likely to
    incur a TLB miss, since the h/w context ID has been changed. The TLB
    will then be reloaded with a translation from the TSB for one process,
    but using a context ID for another process. This exposes memory from
    one process to another, and since it is a mapping for stack memory,
    this usually causes the new process to crash quickly.
    
    This super critical section needs more protection than is provided
    by spin_lock_irqsave() since perf interrupts must not be allowed in.
    
    Since __tsb_context_switch already goes through the trouble of
    disabling interrupts completely, we fix this by moving the secondary
    context load down into this better protected region.
    
    Orabug: 25577560
    Signed-off-by: default avatarDave Aldridge <david.j.aldridge@oracle.com>
    Signed-off-by: default avatarRob Gardner <rob.gardner@oracle.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    fc290a11
tsb.S 13.2 KB