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    SubjectRe: [RFC][PATCH 5/5] tracing/filters: Provide support for char * pointers
    Frederic Weisbecker wrote:
    > Provide support for char * pointers in the filtering framework.
    > Usually, char * entries are dangerous in traces because the string
    > can be released whereas a pointer to it can still wait to be read from
    > the ring buffer. But sometimes we can assume it's safe, like in case
    > of RO data (eg: __file__ or __line__, used in bkl trace event). If
    > these RO data are in a module and so is the call to the trace event,
    > then it's safe, because the ring buffer will be flushed once this
    > module get unloaded.
    >

    The problem is we don't distinguish dangerous char * from
    safe char *... They are both defined as:
    __field(char *, str)

    So for those dangerous ones, a string filter still can be applied,
    which will dereference those pointers.

    > Now the bkl events becomes more useful. Say that you want to trace
    > only the bkl use in reiserfs:
    >
    > cd /debug/tracing/events/bkl/lock_kernel
    > echo "file == fs/reiserfs*" > filter_regex
    > cat /debug/tracing/trace
    >
    > syslogd-3658 [001] 1874.661878: lock_kernel: depth: 1, fs/reiserfs/super.c:563 reiserfs_dirty_inode()
    > syslogd-3658 [001] 1874.662266: lock_kernel: depth: 0, fs/reiserfs/inode.c:2695 reiserfs_write_end()
    > syslogd-3658 [001] 1874.662268: lock_kernel: depth: 1, fs/reiserfs/super.c:563 reiserfs_dirty_inode()
    > syslogd-3658 [001] 1874.662291: lock_kernel: depth: 0, fs/reiserfs/inode.c:2695 reiserfs_write_end()


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