ring.go

Documentation: github.com/cilium/ebpf/ringbuf

     1  package ringbuf
     2  
     3  import (
     4  	"fmt"
     5  	"io"
     6  	"os"
     7  	"runtime"
     8  	"sync/atomic"
     9  	"unsafe"
    10  
    11  	"github.com/cilium/ebpf/internal/unix"
    12  )
    13  
    14  type ringbufEventRing struct {
    15  	prod []byte
    16  	cons []byte
    17  	*ringReader
    18  }
    19  
    20  func newRingBufEventRing(mapFD, size int) (*ringbufEventRing, error) {
    21  	cons, err := unix.Mmap(mapFD, 0, os.Getpagesize(), unix.PROT_READ|unix.PROT_WRITE, unix.MAP_SHARED)
    22  	if err != nil {
    23  		return nil, fmt.Errorf("can't mmap consumer page: %w", err)
    24  	}
    25  
    26  	prod, err := unix.Mmap(mapFD, (int64)(os.Getpagesize()), os.Getpagesize()+2*size, unix.PROT_READ, unix.MAP_SHARED)
    27  	if err != nil {
    28  		_ = unix.Munmap(cons)
    29  		return nil, fmt.Errorf("can't mmap data pages: %w", err)
    30  	}
    31  
    32  	cons_pos := (*uint64)(unsafe.Pointer(&cons[0]))
    33  	prod_pos := (*uint64)(unsafe.Pointer(&prod[0]))
    34  
    35  	ring := &ringbufEventRing{
    36  		prod:       prod,
    37  		cons:       cons,
    38  		ringReader: newRingReader(cons_pos, prod_pos, prod[os.Getpagesize():]),
    39  	}
    40  	runtime.SetFinalizer(ring, (*ringbufEventRing).Close)
    41  
    42  	return ring, nil
    43  }
    44  
    45  func (ring *ringbufEventRing) Close() {
    46  	runtime.SetFinalizer(ring, nil)
    47  
    48  	_ = unix.Munmap(ring.prod)
    49  	_ = unix.Munmap(ring.cons)
    50  
    51  	ring.prod = nil
    52  	ring.cons = nil
    53  }
    54  
    55  type ringReader struct {
    56  	// These point into mmap'ed memory and must be accessed atomically.
    57  	prod_pos, cons_pos *uint64
    58  	cons               uint64
    59  	mask               uint64
    60  	ring               []byte
    61  }
    62  
    63  func newRingReader(cons_ptr, prod_ptr *uint64, ring []byte) *ringReader {
    64  	return &ringReader{
    65  		prod_pos: prod_ptr,
    66  		cons_pos: cons_ptr,
    67  		cons:     atomic.LoadUint64(cons_ptr),
    68  		// cap is always a power of two
    69  		mask: uint64(cap(ring)/2 - 1),
    70  		ring: ring,
    71  	}
    72  }
    73  
    74  func (rr *ringReader) loadConsumer() {
    75  	rr.cons = atomic.LoadUint64(rr.cons_pos)
    76  }
    77  
    78  func (rr *ringReader) storeConsumer() {
    79  	atomic.StoreUint64(rr.cons_pos, rr.cons)
    80  }
    81  
    82  // clamp delta to 'end' if 'start+delta' is beyond 'end'
    83  func clamp(start, end, delta uint64) uint64 {
    84  	if remainder := end - start; delta > remainder {
    85  		return remainder
    86  	}
    87  	return delta
    88  }
    89  
    90  func (rr *ringReader) skipRead(skipBytes uint64) {
    91  	rr.cons += clamp(rr.cons, atomic.LoadUint64(rr.prod_pos), skipBytes)
    92  }
    93  
    94  func (rr *ringReader) Read(p []byte) (int, error) {
    95  	prod := atomic.LoadUint64(rr.prod_pos)
    96  
    97  	n := clamp(rr.cons, prod, uint64(len(p)))
    98  
    99  	start := rr.cons & rr.mask
   100  
   101  	copy(p, rr.ring[start:start+n])
   102  	rr.cons += n
   103  
   104  	if prod == rr.cons {
   105  		return int(n), io.EOF
   106  	}
   107  
   108  	return int(n), nil
   109  }
   110
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