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bpf, libbpf: use correct barriers in perf ring buffer walk
Given libbpf is a generic library and not restricted to x86-64 only, the compiler barrier in bpf_perf_event_read_simple() after fetching the head needs to be replaced with smp_rmb() at minimum. Also, writing out the tail we should use WRITE_ONCE() to avoid store tearing. Now that we have the logic in place in ring_buffer_read_head() and ring_buffer_write_tail() helper also used by perf tool which would select the correct and best variant for a given architecture (e.g. x86-64 can avoid CPU barriers entirely), make use of these in order to fix bpf_perf_event_read_simple(). Fixes:d0cabbb021
("tools: bpf: move the event reading loop to libbpf") Fixes:39111695b1
("samples: bpf: add bpf_perf_event_output example") Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Cc: Peter Zijlstra <peterz@infradead.org> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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@ -27,6 +27,7 @@
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#include <linux/list.h>
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#include <linux/limits.h>
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#include <linux/perf_event.h>
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#include <linux/ring_buffer.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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#include <sys/vfs.h>
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@ -2418,13 +2419,12 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
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unsigned long page_size, void **buf, size_t *buf_len,
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bpf_perf_event_print_t fn, void *priv)
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{
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volatile struct perf_event_mmap_page *header = mem;
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struct perf_event_mmap_page *header = mem;
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__u64 data_head = ring_buffer_read_head(header);
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__u64 data_tail = header->data_tail;
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__u64 data_head = header->data_head;
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int ret = LIBBPF_PERF_EVENT_ERROR;
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void *base, *begin, *end;
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asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
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if (data_head == data_tail)
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return LIBBPF_PERF_EVENT_CONT;
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@ -2467,8 +2467,6 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
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data_tail += ehdr->size;
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}
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__sync_synchronize(); /* smp_mb() */
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header->data_tail = data_tail;
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ring_buffer_write_tail(header, data_tail);
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return ret;
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}
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